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What are Proximity Switches?

By J. Dellaporta
Updated May 17, 2024
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Proximity switches open or close an electrical circuit when they make contact with or come within a certain distance of an object. They are most commonly used in manufacturing equipment, robotics, and security systems. There are four basic types: infrared, acoustic, capacitive, and inductive.

Infrared proximity switches work by sending out beams of invisible infrared light. A photodetector on the switch detects any reflections of this light, which allow the device to determine whether there is an object nearby. As a switch with just a light source and photodiode is susceptible to false readings due to background light, more complex models modulate the transmitted light at a specific frequency and have receivers which only respond to that frequency. Even more complex sensors are able to use the light reflected from an object to compute its distance from the sensor.

Acoustic proximity sensors are similar in principle to infrared models, but use sound instead of light. They use a transducer to transmit inaudible sound waves at various frequencies in a preset sequence, then measure the length of time the sound takes to hit a nearby object and return to a second transducer on the switch. Essentially, acoustic proximity switches measure the time it takes for sound pulses to "echo" and use this measurement to calculate distance, just like sonar.

Capacitive switches sense distance to objects by detecting changes in capacitance around it. A radio-frequency oscillator is connected to a metal plate. When the plate nears an object, the radio frequency changes, and the frequency detector sends a signal telling the switch to open or close. These switches have the disadvantage of being more sensitive to objects that conduct electricity than to objects that do not.

Inductive proximity switches sense distance to objects by generating magnetic fields. They are similar in principle to metal detectors. A coil of wire is charged with electrical current, and an electronic circuit measures this current. If a metallic part gets close enough to the coil, the current will increase and the switch will open or close accordingly. The chief disadvantage of this type is that they can only detect metallic objects.

Proximity switches are used in manufacturing processes, to measure the position of machine components, for example. They are also used in security systems, in applications such as detecting the opening of a door, and in robotics, where they can monitor a robot or its components' nearness to objects and steer it accordingly.

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Discussion Comments
By anon154016 — On Feb 19, 2011

I make table lamps, often used for bedside. Can anyone advise what proximity switch I should use to replace the fiddly switch that comes with bulb-holders. Thanks, Ray in Ledbury, UK

By anon147500 — On Jan 29, 2011

what is the difference between PNP and NPN proximity sensors.

By anon136173 — On Dec 21, 2010

comment 1- It depends on your control circuit or PLC programming. They have NC or NO prox switches.

comment 3- proximity switch and sensor are pretty much the same.

comment 5- It depends on the complexity of your setup or how uniform the part is. But yes you can do that.

comment 11- Proximity sensors detect objects. You would have to used the sensor to control a solenoid valve but not directly either with a PLC or a relay.

comment 13- NPN-sinking PNP- sourcing. NPN basically provides a path to ground and PNP provides a path to +. A NPN sensor is connected to a sourcing PLC and a PNP is connect to a sinking PLC.

By anon121561 — On Oct 25, 2010

What is the difference between NPN and PNP type proximity switches?

By anon99313 — On Jul 26, 2010

we use flow turbines for measurement of flows.We can also use inductive proximity switch at the shaft of a pump to get our flow rate.@ Mkbuser, you need an infrared proximity for that or PIR which uses infra red technology. All depends on what you want to achieve.

By anon95308 — On Jul 12, 2010

what type of proximity switches are we going to use to control the flow of water?

By anon77923 — On Apr 16, 2010

the acoustic proximity switch sounds more like a distance sensing system, rather than a proximity switch.

By anon66683 — On Feb 21, 2010

good info, except what about proximity switches within a computer control program?

By anon53296 — On Nov 20, 2009

Thanks for a very great site. it helps me a lot. very substantial facts. Thanks again.

By anon49106 — On Oct 17, 2009

great work --thanx guys, very informative, consice, to the point - just great!!

By anon49079 — On Oct 17, 2009

We would like to use a proximity switch to limit switch. What kind of modifications to be done? This limit switch was in an electronic circuit.

By anon37085 — On Jul 16, 2009

Can proximity switches be used to detect the configuration of an object? In as much if set up to detect a jar standing up and then a jar came through laying down would this work?

By anon35420 — On Jul 05, 2009

Proximity switch has two states, on/off. Proximity sensor is the sensor used either for proximity switch or for what I could call proximeter which would sense the distance between the sensor and the object.

By anon31063 — On Apr 29, 2009

What is the difference between a proximity switch and a proximity sensor?

By mkbuser — On Sep 20, 2007

What kind of proximity switch would be able to give an alert when something is moving away from it and gets too far away?

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